2021 Equine Infectious Anemia Cases 

in the United States

January 1– December 31, 2021

This document summarizes equine infectious anemia (EIA) cases and testing reported in the United 

States during 2021. Following guidance updates, the over 400 EIA-approved laboratories sent monthly 

summary testing data directly to APHIS Veterinary Services (VS). The National Veterinary Services 

Laboratories (NVSL) confirmed all positive cases, and the VS Equine Health Team compiled case 

information in collaboration with State animal health officials.

EIA test reporting is summarized on a calendar year basis. In 2021, there were 1,416,809 EIA tests 

conducted, resulting in detection of 103 positive horses on 54 premises (Table 1). In comparison, 2020 

results included 1,337,829 tests and 29 positives on 19 premises. Figure 1 displays a map depicting 

reported numbers of horses and premises testing positive for EIA in 2021. Figure 2 presents a summary 

of EIA testing from 2002-2021. Historical data and additional information on EIA are available online at

https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/equine/eia

Table 1. Test results reported for equine infectious anemia in the United States, 2021 

State Tests Performed Positive Horses Positive Premises

Alabama 21,822 1 1

Alaska 602 0 0

Arizona 16,946 5 3

Arkansas 33,398 0 0

California 26,848 1 1

Colorado 25,610 1 1

Connecticut 9,003 0 0

Delaware 3,541 0 0

Florida 104,654 0 0

Georgia 42,154 1 1

Hawaii 438 0 0

Idaho 14,095 0 0

Illinois 26,229 1 1

Indiana 15,379 0 0

Iowa 20,574 1 1

Kansas 15,660 0 0

Kentucky 66,089 0 0

Louisiana 28,975 1 1

Maine 5,565 0 0

Maryland 23,845 0 0

Massachusetts 13,134 0 0

Michigan 33,496 0 0

Animal and Plant 

Health Inspection 

Service 

Veterinary 

Services 

September 2022
2021 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 2 

Minnesota 29,274 0 0

Mississippi 19,017 1 1

Missouri 54,679 10 3

Montana 20,602 0 0

Nebraska 11,000 0 0

Nevada 12,429 0 0

New Hampshire 6,906 0 0

New Jersey 18,845 0 0

New Mexico 14,613 1 1

New York 40,654 0 0

North Carolina 54,239 0 0

North Dakota 7,818 0 0

Ohio 42,459 0 0

Oklahoma 61,035 1 1

Oregon 7,936 1 1

Pennsylvania 48,753 0 0

Rhode Island 1,711 0 0

South Carolina 37,999 0 0

South Dakota 9,784 0 0

Tennessee 53,435 12 2

Texas 181,597 58 30

Utah 9,968 5 3

Vermont 5,739 0 0

Virginia 44,904 2 2

Washington 8,638 0 0

West Virginia 11,901 0 0

Wisconsin 37,076 0 0

Wyoming 15,622 0 0

Puerto Rico 100 0 0

U.S. Virgin Islands 19 0 0

U.S. Totals 1,416,809 103 54
2021 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 3 

Figure 1. Reported numbers of horses and premises testing positive for EIA, 2021 

Although the current prevalence of EIA in the U.S. equine population remains very low, at an estimated 

national level of around 0.004 percent, the epidemiology of EIA-positive cases has shifted in recent 

years. Previously, most EIA cases occurred from natural transmission by biting fly vectors in untested 

and undertested populations; now, there are increasing cases of iatrogenic transmission (a disease 

resulting from medical activity). 
2021 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 4 

Figure 2. Reported numbers of EIA tests and positive cases in the United States, 2002-2021 

In 2021, there were 84 confirmed cases of EIA in either current or former Quarter Horse racehorses

where iatrogenic transmission was determined as the method of spread. Two (2) of these horses were 

racing Paint horses. Iatrogenic transmission in this equine population is occurring through unhygienic

practices by horse trainers and owners. These practices include re-using needles, syringes, and IV sets; 

administering blood transfusions from untested donor horses; using illegal blood products from other

countries; and contaminating multi-dose drug vials with infectious blood. 

Some of the iatrogenic transmission cases in the Quarter Horse racehorse population occur in horses

participating in unsanctioned racing. However, recognized crossover cases between unsanctioned and

sanctioned racing in some parts of the United States also happen. Likewise, there are cases in

sanctioned Quarter Horse racehorses with no apparent connection to unsanctioned racing. EIA-positive 

cases in this emerging high-risk population are frequently found in clusters, indicating a horse trainer or 

owner repeatedly used unhygienic practices that caused the disease spread to multiple horses. These 

cases are preventable by good hygienic practices and basic biosecurity measures. Thus, increased

education and outreach to trainers and owners of Quarter Horse racehorses are necessary to mitigate 

the continued spread of EIA.

Of the remaining 19 positive EIA cases identified in the United States during 2021, 11 originated from

untested/undertested herds where natural transmission was likely occurring over a long period, five (5) 

horse cases involved suspected or confirmed illegal movements into the U.S. from Mexico, and three 

(3) horse cases came from unknown/undetermined sources. 

0

500

1000

1500

2000

2500

Number of Tests x 1000 Number of Test-positive Horses
2021 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 5 

Following changes in APHIS guidance for EIA-approved laboratories, more comprehensive EIA testing 

data is now available. Figure 3 captures the number of EIA tests performed in 2021 by month. April, 

March, and May were the busiest months of EIA testing in 2021. 

Figure 3. Reported numbers of EIA tests performed in the United States in 2021 by month

The two EIA test types authorized for use by approved laboratories in the United States are the agar gel 

immunodiffusion (AGID) tests (commonly called Coggins tests) and the enzyme-linked immunosorbent 

assay (ELISA) tests. Figure 4 depicts the number of AGID tests performed throughout the U.S. in 2021, 

compared to the number of ELISA tests performed. Both test assays were used in 2021 with nearly 

equal frequency. 

Figure 4. Reported numbers of AGID and ELISA EIA tests performed in the United States in 2021 

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000

220000

EIA Tests Performed in 2021 by Month

AGID & ELISA EIA Tests Performed in 2021

AGID Tests

ELISA Tests
2021 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 6 

Figure 5. Reported numbers of AGID and ELISA EIA tests performed by State of horse residence in 2021 

0 20000 40000 60000 80000 100000 120000 140000 160000 180000 200000

Alabama

Alaska

Arizona

Arkansas

California

Colorado

Connecticut

Delaware

Florida

Georgia

Hawaii

Idaho

Illinois

Indiana

Iowa

Kansas

Kentucky

Louisiana

Maine

Maryland

Massachusetts

Michigan

Minnesota

Mississippi

Missouri

Montana

Nebraska

Nevada

New Hampshire

New Jersey

New Mexico

New York

North Carolina

North Dakota

Ohio

Oklahoma

Oregon

Pennsylvania

Puerto Rico

Rhode Island

South Carolina

South Dakota

Tennessee

Texas

Utah

Vermont

Virgin Islands

Virginia

Washington

West Virginia

Wisconsin

Wyoming

AGID & ELISA EIA Tests Performed in 2021 by State of Horse Residence

AGID Tests ELISA Tests
2021 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 7 

Figure 5 shows the number and type of EIA assays used to test resident horses in each State. Twenty-five

States, along with Puerto Rico and the Virgin Islands, reported fewer than 20,000 EIA tests performed on

their horse populations in 2021. Twenty-one States reported between 20,000 and 60,000 EIA tests

performed on their horse populations in 2021. And four (4) States—Florida, Kentucky, Oklahoma, and

Texas—reported over 60,000 EIA tests performed on their horse populations in 2021. Testing on horse

populations in 21 States occurred using an ELISA assay more frequently than the AGID assay. In 

comparison, testing on horse populations in 29 States, as well as Puerto Rico and the Virgin Islands, 

occurred more often using the AGID assay than an ELISA assay.